The present invention relates to a polarization interference microscope for imaging objects. The polarization interference microscope comprises a light source, an illumination beam path, an imaging beam path and an objective. The objective is part of the imaging beam path and may be part of the illumination beam path. The illumination beam path extends from the light source to the object. The imaging beam path extends from the object to a detector or a tube. At least one polarization means is provided in the illumination beam path and/or in the imaging beam path, by which at least one polarization means the light of the respective beam path can be converted into a predeterminable polarization state. An analyzer means is provided in the imaging beam path. A birefringent component is provided between the polarization means and the analyzer means, by which birefringent component the light polarized by the polarization means can be divided into two partial beams having different polarization directions. The component can produce a splitting between the two partial beams. The present invention furthermore relates to a retrofit kit for a conventional microscope and for a conventional polarization interference microscope, respectively, and to a method for imaging objects.
The microscopic examination of objects involves the use of various types of microscopes which are suitable for the respective purpose of application. To examine colourless, transparent and usually very thin objects by means of transmitted light, it is possible to use microscopes which operate according to the interference contrast method. In principle, in microscopes of this type, thickness or height differences in the object are represented by the phase of a plane wave being modulated by the object structure. This modulated wave is then brought to interference with an uninfluenced reference beam. The resultant pattern permits the quantitative determination of phase or path differences in the object. The path differences can also be converted into a plastic or colour-contrasted image in this method.
Alongside the possibility of forming an image from the interference between the modulated wave and an uninfluenced reference beam, there is also the possibility of producing an image with so-called differential interference contrast (DIC). Height differences and material-dependent phase variations at the surface of the object can be represented with high contrast by this method. Contrary to the interference contrast method, in the differential interference contrast method, the modulated wave is not brought to interference with an uninfluenced reference beam, but rather with a laterally offset phase-modulated object wave itself. Therefore, in the differential interference contrast method, the difference values at adjacent object points influence the image generation. Consequently, only those get details are made visible in the immediate vicinity of which there is a refractive index or thickness gradient which can be represented sufficiently by an interference of adjacent waves. Differential interference contrast microscopes usually have, alongside a linear polarizer and an analyzer, at least one wollaston prism by which a splitting and, if appropriate, a recombination of the waves or partial beams is realised.
A microscope that utilizes differential interference contrast mentioned above is known for example from DE 2 401 973 and U.S. Pat. No. 2,601,175. In this case, a condenser prism splits linearly the polarized light into two partial beams which are polarized perpendicular to one another and which are offset parallel to one another. The two partial beams correspondingly pass through the object at different locations and are combined again with the aid of an objective prism arranged downstream of the object. An analyzer arranged subsequently in the beam path brings the two partial beams to interference. Differences in the optical path length which are attributable to height differences or material-dependent phase variations can be converted into intensity differences in this way. A sharp image of the object can then be generated with the aid of said intensity differences.
In principle, this method can also be carried out without the condenser prism. However, the condenser prism is necessary in order to be able to generate a brilliant image, the condenser prism acting as a so-called compensation prism which can be used to compensate for path difference differences of the objective prism on account of the two prism parts.
Contrasting that can be set in a variable manner is not possible, however, with the microscopic imaging methods mentioned above. Such contrasting could be obtained at best by exchanging the prisms provided in the microscope. However, such prisms, as crystal-optical components, are very expensive.
Thus, JP 10161031 A, for example, discloses a variable differential interference contrast in which two wedge-shaped double plates can be displaced with respect to one another for a contrast variation in each on the illumination side and the imaging side. Firstly, this arrangement is extremely complex from a constructional standpoint. Secondly, the splitting elements provided there are situated in a plane conjugate to the object, with the result that, relative to the configurations which are usually used and in which the splitting elements are provided in a pupil plane, for example, additional optical elements are situated between crossed polarizers, which can have a contrast-reducing effect as a result of strains, contaminations and refractions at curved surfaces.